On the shoulders of giants: revisiting phosphate sorption by soils

By Jenny Pham posted 8 days ago

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On the shoulders of giants: revisiting phosphate sorption by soils

The size of the pool of phosphate (PO₄) stored by a soil, and its relation to the PO₄ concentration in solution, are estimated by using PO₄ sorption protocols. These measures are of broad interest as they provide an index of both PO₄ bioavailability and of environmental risk through PO₄ runoff or leaching.

Australian researcher, Dr N Jim Barrow, who passed away in 2025, was a world leader in describing anion sorption by soils. Early in the 1970s he showed that the relation between the concentrations of sorbed PO₄ and those in solution could be fitted by the Freundlich equation: X = a C^b. Here the values of X are the PO₄ concentrations sorbed from multiple PO₄ additions, the C values are the corresponding concentrations of PO₄ remaining in solution, and a and b are coefficients. Jim noted that where sorbed PO₄ was initially present, it depressed subsequent sorption. To quantitatively accommodate this effect, in 1974 he represented the initial sorbed PO₄ by the letter Q, extending the Freundlich equation as follows: X = a C^b + Q.

A statistical difficulty lay festering in the background. That is, uncertainties in the PO₄ additions (probably small), and in the measurements of C (possibly considerable below ~0.5 mg P L^–1), increase uncertainty in the estimates of X, and therefore too those of Q. Jim recognised and addressed this problem using the statistical tools available in 1974. However, statistical approaches have moved on, and we revisited the fitting of the Freundlich equation for PO₄ sorption, extended to accommodate Q, using purpose acquired data.

Our PO₄ sorption observations were made by Natasha Carlson-Perret, as an MSc candidate at the University of Wollongong under the supervision of Professor R John Morrison. Natasha had access to well-characterised soils that had received known additions of PO₄ (Dougherty et al., 2011). She completed the MSc, took a break in the UK, and subsequently completed a PhD at Southern Cross University exploring subtropical seagrass nitrogen cycling.

Joel Tellinghuisen of Vanderbilt University took on the statistical challenge. He concluded that variance weighting of the C values was necessary to improve the quality of the estimates of the Freundlich parameters, and consequently that of Q in the Freundlich equation (Tellinghuisen et al., 2025). Knowledge of the prior PO₄ additions to the soils (Dougherty et al., 2011) was instrumental in testing these inferences. This research makes an important contribution to understanding the behaviour of PO₄ in soils, with implications for the management of soil fertility and environmental risk of PO₄ movement off site, e.g., to waterways (Milham et al., 2025).

Typically, our results question some previous assumptions about the measurement of PO₄ sorption. Two main questions remain open. First, it is likely that molybdate reacts with the PO₄ bound to soluble organic colloids, such as fulvic acid, that are present in the extracts of many soils. Second, it is well-known that molybdate also reacts with AsO₄. That is, it remains unclear how much these two processes may enhance estimates of low, residual PO₄ concentrations (C values) for Australian soils. Such enhancement would prejudice estimates of the coefficients in the Freundlich equation, with negative implications for managing PO₄ in the environment. These unknowns are ripe for investigation.

Supporting literature

Barrow NJ (1974). Effect of previous additions of phosphate on (subsequent) phosphate sorption by soils. Soil Science 118, 82 – 89.
Dougherty WJ, Mason SD, Burkitt LL, Milham PJ (2011). Relationship between phosphorus concentration in surface runoff and a novel soil phosphorus test procedure (DGT). Soil Research 49, 523–528.
Milham PJ, Carlson-Perret N, Morrison R John, Harvey D, Andersson KO, Burkitt LL, Collins D, Haigh AM, Hannah MC, Tellinghuisen J, Dougherty WJ, Holford P (2025). Assessing phosphate sorption by soils: towards resolving the issues. Discover Soils https://doi.org/10.1007/s44378-025-00059-y.
Tellinghuisen J, Holford P, Milham PJ (2025). Analysis of phosphorus soil sorption data: improved results from global least-squares fitting. Soil Systems doi.org/10.3390/soilsystems9010022.